Incidentally CNS tumours may simulate acute bacterial or viral meningitis, cerebral abscess, and tuberculous or luetic basal meningitis. 64 cases from the literature are analysed together with 2 personal observations. This form of presentation is found most frequently in high-grade malignancy, i.e. in glioblastoma, medulloblastoma and ependymoma. In the group of benign CNS neoplasms dermoid and epidermoid cysts are most often associated with the meningeal syndrome. The only criterion facilitating a differentiation between acute bacterial meningitis and CNS malignancy is CSF culture. In individual cases an afebrile course, a normal sedimentation rate, and a normal WBC count may help to differentiate. In the presence of basal meningitic or diencephalic symptoms related to a prolonged course discrimination between brain tumour an tuberculous or luetic meningitis may become extremely difficult. In these cases the determination of creatine kinase BB isoenzyme and carcinoembryonic antigen in CSF may overcome the difficulty.